Spray pack diffusion coatings for refractory metals



United States Patent 3,514,315 SPRAY PACK DIFFUSION COATINGS FOR REFRACTORY METALS Richard A. J efferys, Euclid, and John D. Gadd, Willowiclt, Ohio, assignors to TRW Inc., Cleveland, Ohio, :1 corp0- ration of Ohio No Drawing. Filed Sept. 26, 1966, Ser. No. 581,733 Int. Cl. C23c 11/02 US. Cl. 117-71 9 Claims ABSTRACT OF THE DISCLOSURE Process for providing a protective coating on columbium or tantalum based metals which involves applying a slurry of a coating composition including chromium, titanium, an inorganic halide activator, and a thermally depolymerizable resin over the surface to be treated, and heat treating the article under non-oxidizing conditions until the chromium and titanium are diffused into the refractory metal surface. This may be followed by a similar application of a silicon slurry.

The present invention relates to the coating of refractory metals, particularly metals such as columbium and tantalum, and alloys containing at least 50% by weight of such metals. The metals columbium and tantalum have inherently poor oxidation resistance which precludes their utilization in high temperature oxidizing environments despite their substantial strength potential.

One of the applicants herein, in prior filed applications Ser. No. 23,239 (filed Apr. 19, 1960, now abandoned) and copending Ser. No. 449,370 (filed Apr. 9, 1965, now abandoned) has disclosed and claimed methods for providing such metals with increased resistance to thermal shock, impact, abrasion, ablation and creep. Basically, the methods described in the aforementioned applications involve a pack diffusion process wherein the article to be coated is packed in a powdered mixture of chromium and titanium metals which is then positioned in a vacuum chamber and heated to a temperature sufficient to vaporize the powdered coating material, thereby causing a solid-vapor interface reaction to occur between the base metal and the coating vapor. The processes described in said applications produce highly acceptable oxidation protected columbium surfaces, but the times involved and the equipment required are quite substantial. In this type of process, the retort, the pack materials, and the articles to be coated must all be heated to the coating temperature. This takes a considerable period of time since heat transfer through the granular media is poor. For the same reasons, the furnace had to be cooled for quite a long time before the articles could be removed. Furthermore, it was not feasible, with the solid pack assembly to coat only selected areas of the refractory article where such would be desired.

One of the objects of the present invention is to provide a more simplified diffusion coating technique for refractory metals such as columbium and tantalum.

Another object of the invention is to provide a diffusion coating technique which is far less time consuming than the solid pack technique, but achieves results which are substantially comparable.

Another object of the invention is to provide a method for coating columbium and the like which allows for faster heating and cooling cycles in the furnace.

Still another object of the invention is to provide a method for coating columbium articles which can be applied to selected areas of the article, when desired.

Another object of the invention is to provide a coating ice process which lends itself to coating large or complex refractory metal structures.

In accordance with the present invention, we provide a vapor-solid interface reaction analogous to that achieved in the pack type process with a coating type process wherein a slurry containing chromium and titanium is first applied to the columbium article as an adherent coating. The original application is preferably carried out by spraying the surface of the article to be coated with a conventional spray gun. After the desired thickness has been built up, the coating is air dried to form a bisque with adequate handling strength. Next, diffusion treatment of the sprayed material is carried out under nonoxidizing conditions, preferably in the presence of an inert gas such as argon or the like at subatmospheric pressure or in vacuum. The coating which results may range from a simple overlay of chromium and titanium over the columbium surface, or under ideal circumstances, the titanium and chromium appear as a uniformly thick overlay coating with an intermediate diffusion reigon in the adjacent substrate between the overlay and the columbium metal. The diffusion region apparently includes a solid solution of columbium, chromium, and titanium. The optimum microstructure for the chormium-titanium coating consists of a continuous Laves phase overlay measuring from about 0.8 to 1.0 mil and a metallographically evident diffusion region measuring from about 1.0 to 1.2 mils.

Following the initial heat treatment to form the chromium-titanium overlay, the coated article is then preferably subjected to a second diffusion treatment utilizing metallic silicon as the coating agent. The silicon can also be applied as a slurry, dried, and heat treated under nonoxidizing conditions to cause penetration of the silicon into the chromium-titanium layer and the formation of a chrornium-silicon-titanium alloy or mixture usually measuring from 3.0 to 3.5 mils in thickness.

The coating composition applied to the refractory surface to be protected consists of the chromium-titanium mixture, an inorganic activator such as potassium fluoride or sodium fluoride, and an organic binder which is preferably a thermally depolymerizable polyolefin such as polyisobutylene, polybutene, or the like which decom poses at elevated temperatures without leaving a carbonaceous residue. The mixture of metal powders, inorganic activator, and binder usually include amounts of to by weight of the metal powders, about 5 to 12% by weight of the binder, and from about 1 to 5% by weight of the halide activator. A sufficient amount of a readily volatilizable vehicle such as toluene is added to attain a satisfactory spraying viscosity.

The best results have been obtained in the practice of the present invention by using, as the metal powder con stituent of the first slurry, a mixture of a pre-alloyed chromium-titanium alloy and metallic titanium. Of all these combinations which we have tested, we have obtained the best results by using a mixture of about 90% by weight of a 50-50 alloy of chromium and titanium in combination with about 10% by weight of metallic titanium as the metallic constituent of the slurry. Coatings produced with this type of composition provided a more uniformly thick overlay coating with a more apparent diffusion region in the adjacent substrate.

In addition to the use of the mixture of a pre-alloy of chromium and titanium with titanium powder, fairly good results are obtained by using a mixture of chromium powder and titanium powder in relatively pure form, the mixture containing about 80 to chromium and 5 to 20% by weight titanium.

The article to be coated is first degreased as by treatment in trichlorethylene, and then abrasively tumbled to remove burrs. Next, the samples may be chemically etched in an aqueous solution of hydrofluoric, sulfuric, and nitric acids. The articles are then subsequently sprayed with a slurry of the type described, and built up to a thickness of 2 to 10 mils or so. Upon air drying, the slurry forms a bisque having adequate handling strength.

The diffusion treatment of the bisque coated articles can be accomplished in an induction heated vacuum furnace by packing the specimens in a refractory metal retort partially sealed with a getter composition containing 50% chromium and 50% titanium alloy granules. While the heat treatment conditions can vary considerably depending upon the specific composition employed, the first heat treatment step is carried out normally at a temperature of about 2000 to 2500 F. for from 3 to 10 hours.

The silicon is applied in the same type of slurry as the titanium and chromium, that is, one containing an inorganic activator and an organic binder. The heat treatment temperature in the second stage ranges from about 1900 to 2200 F. for periods of time ranging from about 1 to hours.

The siliconizing cycle can best be carried out using a slurry containing 100% silicon particles at a particle size of -250 mesh and a diffusion environment of argon at an absolute pressure of 150 millimeters of mercury. The siliconizing slurry may contain 85-90% silicon, 5- 12% depolymerizable organic binder, and 15% of the halide activator.

Much of the experimental work done in connection with the present invention was carried out on columbiurn base alloys known as D-43 and B66, having typical analyses illustrated in the following table:

TABLE I Constituent The following table sets forth the metallographic and cyclic oxidation test results obtained by siliconizing under various process conditions a base material consisting of a chromium-titanium coated B-66 alloy having a chromium-titanium coating thickness ranging from 0.8 to 1.0 mil, and a diffusion zone of 1.0 to 1.2 mils. The chromium-titanium coating slurry contained 89% by weight of metal powders consisting of 90 parts of a 50-50 Cr-Ti alloy and parts titanium powder, 1% sodium fluoride and 10 polyisobutylene binder as the solids. The siliconizing slurry contained 89% by Weight of pure silicon, 1% potassium fluoride and 10% polyisobutylene binder as the solids. Both diffusion treatments took place in argon at a pressure of 150 mm. mercury. The samples were tested at temperatures ranging from 1800 to 3000 F. by exposing them to oxidation in resistance heated box furnaces. At temperatures of 1800 to 2800 F., the test coupons were supported on either high purity alumina or fused quartz. At temperatures on the order of 3000 F., sacrificial molybdenum pads protected with a silicontungsten coating were employed as the support media. The test involved thermal cycling of the samples with air cooling to approximately room temperature for visual inspection. The exposure interval between cycles was decreased with increasing test temperature for improved accuracy in detecting the time of coating failure.

The test results obtained are given in the following table, four samples belng tested at each set of conditions:

TABLE II siliconizing conditions Cr-Ti-Si coating thick- Protective life F. ness, mills hours at 2,500 F.

1, 900 1. 2-1.3 53, 55, 72,72. 1, 900 2. 2-2. 5 94,94,9e120. 1, 900 2. 7-3. 0 96, 96, 96, 128. 2, 000 1. 9-2. 0 53, 72, 96, 120. 2, 000 3. 0-3. 4 120, 120, 120, 128. 2, 000 3. 6 1. 0 120, 120,128,128. 2, 100 2. 4-2. 8 96, 120, 120, 128. 2, 100 4.0-4.3 All four above 150. 2,100 3. 6-4. 4 72,128,128,128.

Based upon tests such as the foregoing, we have concluded that the optimum conditions for the formation of chromium-titanium-silicon coatings of 3.0 to 3.5 mil thicknesses on chromium-titanium coated columbium substrates involved the use of 100% silicon in the slurry, an activator consisting of 1.0 weight percent sodium fluoride, and a bisque thickness of 10 mils minimum. The preferred diffusion environment consists of an atmosphere of 150 millimeters of argon pressure at a temperature of 2000 F. for a diffusion time of 3 hours.

The following table lists some tensile properties of the D-43 alloy when provided with a chromium-titaniumsilicon coating according to the techniques described herein:

TABLE III 0.2% Ultimate offset Percent tensile yield elong. Temstrength, strength, in 1 Percent perature p.s.i. p.s.i. gage R.A

Room 72, 900 51, 400 25. 3 55. 3 72, 300 50, 700 27. 0 67. 0 72, 800 51, 800 25. 4 51. 3 2000 F- 32, 300 28, 000 17. 4 27. 4 32, 700 28, 700 19. 0 29. 6 33, 000 29, 400 18. 4 30. 0 2500 F" 17, 800 15, 600 61. 9 80. 7 16, 400 13, 900 76. 3 76. 5

The following table sets forth some room temperature tensile properties of the D-43 alloy sheet coated with the chromium-titanium-silicon combination of the present invention, after various creep test exposures:

TABLE IV Creep test parameters Tensile Properties Creep, Percent o percent U.T.S., 0.2 offset elong. in Percent Temp. F. Stress, p.s.1. Hrs elong. p.s.i. Y.S., psi. 1 gage R.A.

The coating techniques of the present invention are also applicable to the coating of tantalum base materials. While such techniques are applicable to the coating of tantalum, experience has shown that the results obtained are not as good as those obtained with columbium. The lesser protective performance can be attributed in part to the slower diffusion of chromium and titanium in the tantalum matrix, in contrast to columbium, and an attendant absence of the required oxidation resistant solid solution region, i.e., the diffusion Zone beneath the chr0- mium-titanium-si1icon overlay.

It should be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

We claim as our invention:

1. The method of providing a protective coating on a surface of a refractory metal selected from the group consisting of columbium and tantalum based metals which comprises applying a slurry of a coating composi tion containing chromium and titanium over said surface, said slurry also containing an inorganic halide activator and a thermally depolymerizable polyolefinic resin which volatilizes without leaving a carbonaceous residue, and heat treating the resulting coated article under non-oxidizing conditions until the chromium and titanium diffuse into the refractory metal surface.

2. The method of claim 1 in which said activator is sodium fluoride.

3. The method of providing a protective coating on a surface of a columbium base metal which comprises applying a slurry of metal powders including chromium and titanium over said surface to form an adherent coating thereon, said slurry of metal powders also including an inorganic halide activator and a thermally depolymerizable polyolefinic resin which volatilizes without leaving a carbonaceous residue, heat treating the resulting coated article under non-oxidizing conditions to cause diffusion of chromium and titanium into said surface, coating the resulting article with a second slurry contain ing metallic silicon therein, said second slurry also including an inorganic halide activator and a thermally depolymerizable polyolefinic resin which volatilizes with out leaving a carbonaceous residue, and heating the result ing coated article under non-oxidizing conditions to pro duce a diffusion layer on said article containing chromium, titanium and silicon.

4. The method of claim 3 in which the first named slurry contains a mixture of pre-alloyed chromiumtitanium alloy and metallic titanium.

5. The method of claim 3 in which the first heat treating step is carried out at a temperature of between 2000 and 2500 F. from 3 to 10 hours.

6. The method of claim 3 in which the first named slurry contains to by weight of metal powders, about 5 to 12% by Weight of an organic thermally depolymerizable binder, and 1 to 5% by weight of an inorganic halide activator.

7. The method of claim 3 in which the silicon containing slurry contains 85 to 90% by weight of silicon, about 5 to 12% by weight of an organic thermally deploymerizable binder, and about 1 to 5% by weight of an inorganic halide activator.

8. The method of claim 3 in which the second heat treating step is carried out at a temperature between 1900 and 2200 F. for a time of from 1 to 5 hours.

9. The method of claim 3 in which the first named slurry contains a mixture of about 90% by weight of a 5050 alloy of chromium and titanium, and about 10% by weight of metallic titanium.

References Cited UNITED STATES PATENTS 2,855,332 10/1958 Samuel.

2,858,600 11/1958 Vigor 117--l31 X 3,037,883 6/1962 Wachtell et al.

3,061,462 10/1962 Samuel.

3,102,044 8/1963 Joseph 117--131 X 3,317,343 5/1967 Jefferys.

3,418,144 12/1968 Culp et a1 117106 X OTHER REFERENCES Klopp, W., Review of Recent Developments on Oxidation-Resistant Coatings for Refractory Metals in United States Department of Commerce, Office of Technical Services, OTS 61-674.

ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner U.S. Cl. X.R. 

